A spark plug for providing ignition in an internal combustion engine, such as an automobile engine, generally includes a tubular metallic shell; a tubular insulator disposed in the bore of the metallic shell; a center electrode disposed in a forward end portion of the axial bore of the insulator; a metal terminal disposed in a rear portion of the axial bore; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end faces the center electrode, thereby forming a spark discharge gap therebetween. When high voltage is applied between the center electrode and the metallic shell, spark discharge is generated between the center electrode and the ground electrode and ignites fuel contained in a combustion chamber.
At the time of spark discharge, if the insulator has a defect, such as a fine pore, current may leak, through the defect, between the center electrode and the metallic shell (hereinafter, the current leakage may be referred to as penetrating discharge), potentially resulting in a failure to perform normal spark discharge.
In order to provide a method of readily and reliably detecting a defect in a ceramic insulator, Japanese Patent No. 2550790 (“Patent Document 1”) describes “a defect detection method for a ceramic insulator, characterized in that: while the ceramic insulator having a hollow portion whose one end is open is held, spark discharge is generated between a first electrode inserted into the hollow portion of the ceramic insulator and a second electrode provided in the vicinity of ceramic insulator; a check is made to see whether spark discharge generated through generation of a voltage difference between the first and second electrodes has passed through the opening of the hollow portion of the ceramic insulator; and whether or not the ceramic insulator is acceptable is judged from whether or not the spark discharge has passed through the opening of the hollow portion” (claim 5 of Patent Document 1). According to this method, in the case where spark discharge is generated between the first and second electrodes while passing through the opening of the hollow portion of the ceramic insulator, the ceramic insulator is judged free of defect. In the case where spark discharge is generated between the first and second electrodes without passage through the opening of the hollow portion of the ceramic insulator, the ceramic insulator is judged defective, since penetrating discharge has been generated through passage through a defect in the ceramic insulator.
Meanwhile, in the above-mentioned defect detection method, in order to detect a finer defect in the ceramic insulator, it is effective to apply a higher voltage; i.e., a greater electric potential difference, between the first and second electrodes. This is for the following reason: in the case where the ceramic insulator has a defect, penetrating discharge is more likely to be generated through the defect. Thus, when the above-mentioned defect detection method is employed, increasing the electric potential difference between the first and second electrodes is conceived. However, even though applied voltage is increased, when applied voltage reaches a predetermined value, spark discharge is generated through the opening of the hollow portion. Therefore, application of voltage equal to or higher than the predetermined voltage is meaningless; i.e., there is a limit to mere application of high voltage for improvement of accuracy in detecting a fine defect in the ceramic insulator.
In order to cope with such a problem, Japanese Patent Application Laid-Open (kokai) No. 2004-108817 (“Patent Document 2”) describes “a defect detection method for a ceramic insulator, characterized in that: . . . while the interior of the pressure vessel is sealed with air having a pressure higher than the atmospheric pressure, an electric potential difference is generated between the first electrode and the second electrode to thereby detect a leak current which flows between the first electrode and the second electrode, and whether or not the ceramic insulator has a defect is judged from whether or not the leak current is greater than a predetermined value” (claim 1 of Patent Document 2). According to the conception of this invention, by means of the interior of the pressure vessel being sealed with air having a pressure higher than the atmospheric pressure, aerial spark discharge between the first electrode and the second electrode is restrained, whereby there can be increased an electric potential difference required to generate spark discharge which passes through the hollow portion. Thus, while the generation of spark discharge which passes through the hollow portion is restrained, the electric potential difference between the two electrodes can be increased. As a result, there is improved accuracy in detecting a fine defect in the ceramic insulator.